Regardless of the source of a ballistic projectile, a ballistic projectile can puncture the fuel tank which results not only in damage to the fuel tank, but may also allow fuel to leak from the tank and increase the possibility of a fire or explosion. Moreover, a ballistic projectile that enters a fuel tank may also create a hydrodynamic ram effect which, in turn, can produce even larger holes and tears in a fuel tank and, accordingly, an even greater risk of fire or other damage. In this regard, a ballistic projectile that penetrates a fuel tank has a large amount of kinetic energy. As the projectile passes through the fuel in the tank and is slowed by the fuel, the kinetic energy of the ballistic projectile is transferred to the fuel as a pressure wave. The resulting pressure wave may then strike the wall of the fuel tank over a relatively large area and, depending upon the magnitude of the pressure wave and the construction of the fuel tank, may create a relatively large hole or tear in the wall of the fuel tank.
Various approaches have been taken to protect fuel tanks from ballistic projectiles and/or to minimize the damage created by ballistic projectiles. For example, fuel tanks have been shielded with protective armor to prevent or at least reduce the number of ballistic projectiles that penetrate the fuel tanks. However, the additional weight necessitated by the armor is disadvantageous for vehicular applications including, in particular, aircraft applications in which weight has a direct effect upon the performance of the vehicle and its operational costs. In order to reduce the likelihood of a fire or explosion in the event of the puncture of a fuel tank by a ballistic projectile, fire extinguishing systems have also been employed. These fire extinguishing systems are designed to flood the fuel tank with either an inert gas, such as nitrogen, or a fire extinguishing foam. The use of fire extinguishing systems also disadvantageously increase the weight of the vehicle, and while the fire extinguishing systems may reduce the likelihood of a fire or other explosion, these fire extinguishing systems do not generally prevent the leakage of fuel from punctured fuel tanks.
Additionally, a self-sealing bladder has been disposed within fuel tanks in an effort to limit any fuel spill that would otherwise result from the impact of a ballistic projectile with the fuel tank and, accordingly, to similarly limit the risk of fire or explosion occasioned as a result of fuel leakage. A self-sealing bladder may consist of three layers of rubber with the inner and outer layers being fuel-resistant rubber barriers and the middle layer being thicker and formed of natural rubber. When punctured by a ballistic projectile or otherwise, the middle layer of the bladder will come into contact with the fuel and swell. This swelling of the middle layer of the bladder will seal a hole or tear if the hole or tear is relatively small, thereby limiting the fuel that will spill from the tank in such instances. While self-sealing bladders have been useful, self-sealing bladders add weight to a vehicle which, at least in the instance of an aircraft, may reduce the payload that the aircraft is capable of carrying and/or reduce the range of the aircraft. Additionally, while self-sealing bladders may seal a relatively small hole or tear, at least some fuel generally leaks through the bladder and out from the tank prior to the swelling of the middle layer of the bladder and the sealing of the hole or tear. As such, the fuel that leaks from the tank prior to sealing of the hole or tear still presents at least a potential fire hazard. Further, the hydrodynamic ram protection afforded to the fuel tank by such self-sealing bladders is relatively limited such that larger holes or tears may be created by the hydrodynamic ram effect in some circumstances, even though the fuel tank may be lined with a self-sealing bladder.
Accordingly, while self-sealing bladders and other techniques have at least partially addressed issues associated with the damage to fuel tanks and the threat created by fuel spills from a damaged fuel tank, it would be desirable to provide improved techniques for limiting the damage occasioned by a ballistic projectile impacting a fuel tank, both in terms of the hole or tear created by the ballistic projectile and any resulting fuel spillage from such hole or tear, also in terms of the limitation or prevention of further damage to the fuel tank due to the hydrodynamic ram effect created by a ballistic projectile within the fuel tank.